1. Field of the Invention
This invention relates generally to the field of geophysical prospecting. More particularly, the invention relates to the field of imaging dual-sensor marine seismic streamer data.
2. Description of the Related Art
In the oil and gas industry, geophysical prospecting is commonly used to aid in the search for and evaluation of subsurface earth formations. Geophysical prospecting techniques yield knowledge of the subsurface structure of the earth, which is useful for finding and extracting valuable mineral resources, particularly hydrocarbon deposits such as oil and natural gas. A well-known technique of geophysical prospecting is a seismic survey. In a land-based seismic survey, a seismic signal is generated on or near the earth's surface and then travels downward into the subsurface of the earth. In a marine seismic survey, the seismic signal may also travel downward through a body of water overlying the subsurface of the earth. Seismic energy sources are used to generate the seismic signal which, after propagating into the earth, is at least partially reflected by subsurface seismic reflectors. Such seismic reflectors typically are interfaces between subterranean formations having different elastic properties, specifically sound wave velocity and rock density, which lead to differences in acoustic impedance at the interfaces. The reflected seismic energy is detected by seismic sensors (also called seismic receivers) at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes. The seismic sensors generate signals, typically electrical or optical, from the detected seismic energy, which are recorded for further processing.
The appropriate seismic sources for generating the seismic signal in land seismic surveys may include explosives or vibrators. Marine seismic surveys typically employ a submerged seismic source towed by a ship and periodically activated to generate an acoustic wavefield. The seismic source generating the wavefield may be of several types, including a small explosive charge, an electric spark or arc, a marine vibrator, a water gun, a vapor gun, and, most typically, an air gun. Typically, a marine seismic source consists not of a single source element, but of a spatially-distributed array of source elements. This arrangement is particularly true for air guns, currently the most common form of marine seismic source.
The appropriate types of seismic sensors typically include particle velocity sensors, particularly in land surveys, and water pressure sensors, particularly in marine surveys. Sometimes particle displacement sensors, particle acceleration sensors, or pressure gradient sensors are used in place of or in addition to particle velocity sensors. Particle velocity sensors and water pressure sensors are commonly known in the art as geophones and hydrophones, respectively. Seismic sensors may be deployed by themselves, but are more commonly deployed in sensor arrays. Additionally, pressure sensors and particle motion sensors may be deployed together in a marine survey, collocated in pairs or pairs of arrays.
In a typical marine seismic survey, a seismic survey vessel travels on the water surface, typically at about 5 knots, and contains seismic acquisition equipment, such as navigation control, seismic source control, seismic sensor control, and recording equipment. The seismic source control equipment causes a seismic source towed in the body of water by the seismic vessel to actuate at selected times. Seismic streamers, also called seismic cables, are elongate cable-like structures towed in the body of water by the seismic survey vessel that tows the seismic source or by another seismic survey ship. Typically, a plurality of seismic streamers are towed behind a seismic vessel. The seismic streamers contain sensors to detect the reflected wavefields initiated by the seismic source and reflected from reflecting interfaces. Conventionally, the seismic streamers contain pressure sensors such as hydrophones, but seismic streamers have been proposed that contain water particle velocity sensors such as geophones or particle acceleration sensors such as accelerometers, in addition to hydrophones. The pressure sensors and particle motion sensors may be deployed in close proximity, collocated in pairs or pairs of arrays along a seismic cable. An alternative to having the geophone and hydrophone co-located, is to have sufficient spatial density of sensors so that the respective wavefields recorded by the hydrophone and geophone can be interpolated or extrapolated to produce the two wavefield signals at the same location.
After the reflected wave reaches the streamer cable, the wave continues to propagate to the water/air interface at the water surface, from which the wave is reflected downwardly, and is again detected by the hydrophones in the streamer cable. The water surface is a good reflector and the reflection coefficient at the water surface is nearly unity in magnitude and is negative in sign for pressure signals. The waves reflected at the surface will thus be phase-shifted 180 degrees relative to the upwardly propagating waves. The downwardly propagating wave recorded by the receivers is commonly referred to as the surface reflection or the “ghost” signal. Because of the surface reflection, the water surface acts like a filter, which creates spectral notches in the recorded signal limiting the bandwidth of the recorded data. Because of the influence of the surface reflection, some frequencies in the recorded signal are amplified and some frequencies are attenuated.
A particle motion sensor, such as a geophone, has a directional response, whereas a pressure sensor, such as a hydrophone, does not. Accordingly, the upgoing wavefield signals detected by a geophone and hydrophone located close together will be in phase, while the downgoing wavefield signals will be recorded 180 degrees out of phase. Various techniques have been proposed for using this phase difference to reduce the spectral notches caused by the surface reflection. Conventional techniques for deghosting often include combining the pressure and vertical particle velocity wavefields to separate one of the pressure or vertical particle velocity wavefields into at least one of up-going and down-going wavefield components.
The measurements made by motion sensors in towed streamer cables for measuring the particle motion associated with pressure waves are vector measurements. Therefore, unlike pressure measurements, the recorded amplitudes are dependent upon the incidence angle relative to the vector measurement direction. If the vertical velocity field is being measured, then the recorded amplitudes are proportional to the cosine of the incidence angle relative to the vertical. With single component motion sensors, this angle dependency has to be corrected for before the velocity field can be combined with the total pressure field to separate up-going and down-going wave-fields. One way of doing this angle dependent amplitude correction is to decompose the measured data into plane-waves, and then divide the amplitudes by the cosine of the angle of each plane wave. At a zero degree emission angle, the measurement direction is in the same direction as the particle motion, and no correction is needed to the amplitudes after decomposing into plane-waves.
There are several limitations with this conventional method. The method requires data that are densely sampled spatially, in both the in-line and cross-line directions, in order to avoid aliasing in the plane-wave decomposition. In addition, the signal to noise ratio tends to decrease with increasing incidence angles. The signals of interest decrease in amplitude with increasing angle as described above, whereas noise related to mechanical vibrations in the streamers does not follow the same angle dependency as the signal because the noise propagate with a slower velocity along the streamer compared to acoustic energy, and aliasing tends to occur at relatively low frequencies. Such noise tends to be spread over the entire angle range of interest. Hence, the signal level relative to the noise level tends to decrease with increasing angle.
Thus, a need exists for a method for separating the pressure or vertical velocity fields into at least one of up-going and down-going wavefield components without requiring dense spatial sampling, especially in the cross-line direction, and without requiring knowledge of the incidence angles.